I picked up a 1987 Polaris Indy Trail 440 fan to bomb around with the kids. Problem is that it backfires when I hit over 6000 rpm's like a pop, pop, pop. seems to bog a little at the same time but still has some power. I am not familiar working on sleds but can do stuff myself. So far I replaced the plugs, took the carb's apart and cleaned them. It iddled high and adjusted it down to about 2000 rpm. If the track if off the ground it does not really backfire but when I am on it, clutch engages around 4000 rpm and coasting at 5000 rpm's I am not moving fast but it is ok. Give it gas to over 6000 rpm's and she backfires like a sputter. Any clue as to what I should check?